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2 nd International Conference and Exhibition on 16 Giugno 2014 Lasers, Optics & Photonics, Philadelphia 8 -10 Sept, 2009 Liquid droplet whispering-gallery-mode optical resonators Gianluca Gagliardi National Institute of Optics (INO), Italian Research Council Napoli, ITALY Via Campi Flegrei 34, 80078 Pozzuoli (NA) Tel. +39 081 8675423 – email: gianluca. gagliardi@ino. it
Outline Ø Introduction to WGMs Ø WGM of liquid droplets in the near IR Ø Laser frequency locking to WGMs Ø Cavity ring-down spectroscopy in droplet resonators Ø Wavelength-shift tracking for dissolved particles Ø WGMs detection of NPs in the visible Ø Conclusions and outlook
Whispering gallery mode (WGM) resonators Total internal reflection SCHIBLI−LAB AT THE UNIVERSITY OF COLORADO Resonance condition • Solution: spherical Bessel fun N, spherical harmonics l, m • Modes: angular momentum num l, projection m N number of radial maxima Fundamental equatorial modes • Analogy between WGMs and acoustic waves • Smallest volume, lowest loss (n=1, l max, m=±l) • Light field confined near the sphere surface • EW into the external medium
WGM resonators Fused silica: Gorodetsky et al. , OL 21, 453 (1996). Ca. F 2: Savchenkov et al, PRA 70, 051804 (2004); OE 15, 6768 (2007). Grudinin et al Opt. Commun. 265, 3338 (2006). Solid H 2: K. Hakuta et al. OL 27 (2002) LN WGM: D. A. Cohen and A. F. J. Levi, Electron. Lett. 37 (1) , 2001. LN WGM resonator, D. Haertle, T. Beckmann, J. Schwesyg, S. Hermann, A. Zimmermann, K. Buse Photonics West 2009 Fused silica: K. Vahala et al. , Nature 421, 925 (2003) Ca. F 2 Qm>105: J. Hofer, A. Schliesser, P. Del’Haye, and T. Kippenberg (CLEO’ 09) Mg. F 2 resonator, OEwaves (2011) Si: Appl. Phys. Lett. 85 (2004) Mg. F 2 resonator, C. Y. Wang, T. Herr, P. Del'Haye, A. Schliesser, J. Hofer, R. Holzwarth, T. W. Hänsch, N. Picqué, T. J. Kippenberg, ar. Xiv: 1109. 2716 BBO resonator, JPL, 2012 Fabricating high Q spherical microresonators is very easy… SBN resonator, OEwaves (2012) 6
Spherical micro-resonators as sensors J. A. Barnes, G. Gagliardi, H. -P. Loock, Optica 1 (2), 75 (2014) (>109) finesse Loss mechanisms: – Radiative loss – Material absorption – Surface scattering – Coupling loss (prism, tapers) Whispering-gallery modes in solid cavities • Long-decay time, high Q (although…) • Sensitive detection on small volumes • Silica MRs used mostly for “refractive” sensing • Optical coupling: prisms, fiber tapers… • WGM: EW tail outside the sphere • Few works on gas sensing with CEAS • Liquid sensing difficult
Liquid droplet resonators: motivations • Optical cavities with relatively-high Q-factor ( 105 -107) • Analyte or particles inside resonator • Full interaction with WGM mode, not only evanescent field • Ease of fabrication • How to couple light into a liquid resonator • How to handle it • Deal with evaporation if water…
Free-space WGM coupling: preliminary Solid PDMS He-Ne Laser Ex: Solid PDMS sphere (dia ~ 1. 5 mm) Problems to droplet cavities: • Alignment • How to handle the drop: -hydrophobic/hydrophilic surface?
WGMs in liquid drop resonators Optical fiber liquid paraffin Microscope objective Laser beam ~ 1 mm Resonance spectra observed by a current-modulated DBR Laser at 1083 nm Photodetector
Optical configuration for free-space coupling • Q 105 • Mechanically robust • Highly reproducible • Diameters from 200 to 1400 m
WGMs spectra at 1560 nm TE & TM WGMs IR camera view
WGMs spectra at 1560 nm Modes are not degenerated Ellipticity of the droplet g Degeneracy lifting due to ellipticity e = (rpol − req)/R,
S. Avino, A. Krause, R. Zullo, A. Giorgini, P. Malara, P. De Natale, H. -P. Loock, and G. Gagliardi, Adv. Opt. Mat. , in press
Laser-frequency locking to WGMs Locking the laser emission frequency to the cavity modes allows -Real-time tracking of the resonance -No need for laser scan -Reduced need for fitting or post-processing -WGM shift measurements below the HWHM possible -static or dynamic measurements possible Very-low noise techniques can be used to extract a suitable signal for locking & measure tiny effects in the cavity… (G. Gagliardi e al. , Science 330, 1081 -1084 (2010))
The Pound-Drever-Hall technique • Laser frequency detuning is measured vs resonance • The measured signal is fed-back to the laser (or the cavity) • Null detection – total decoupling from the laser intensity noise • Intrinsic low-noise measurement - high frequency modulation • High bandwidth lock E. D. Black, Am. J. Phys. 69, 79 (2001)
PDH locking signals for WGMs Scatter PDH from transmission high S/N Active stabilization of the laser on a single WGM …even below the mode linewidth
Laser-frequency locking to the resonance From the FFT of the error signal: Laser noisefloor
Cavity ring-down spectroscopy (CRDS) Amp modulated beam empty-cavity single-mode decay signals absorbing medium Time Absorption coefficient Attenuated & -shifted beam Time-domain CRDS: • Direct absorption coeff • Fast detect • Noise immune
Lock & Detection scheme S. Avino, A. Krause, R. Zullo, A. Giorgini, P. Malara, P. De Natale, H. -P. Loock, and G. Gagliardi, Adv. Opt. Mat. , in press
WGM cavity ring-down signals Once the laser is locked to the cavity mode, a square-wave AM is applied: The photon lifetime is very short: very fast response/transition times are required Build-up & Ring-down
WGM: t-cavity ring-down & linewidth 683 2 MHz A = 232 1 ps - Good consistency between the linewidth and photon lifetime - Minimum detectable abs coeff change 7 10 -5 mm-1
CRDS: decay signals for different oils = 166. 5 0. 4 ps = 367. 0 0. 4 ps
Oil contamination: CRD vs. concentration Seeds oil in olive oil with 0, 33, 66 and 100 % concentration Good agreement with the measured abs cofficients of 2 components
Droplet real-time response to mechanical stimuli Pulsed excitation Slow perturbation Calibration of real-time WGM shift The feedback signal exhibits a fast response High S/N Limited by ambient noise
Prospect for NP detection • Laser frequency noise sets the ultimate limit on short time scales: 30 k. Hz/ Hz 0. 2 fm/ Hz on a ms scale 2 k. Hz/ Hz < 10 -2 fm/ Hz on a s scale • Ambient noise affects the low-frequency range: 15 fm/ Hz on a s scale
Q-factor considerations • In droplet cavities the dominant loss can be due to liquid Ex. In the NIR, Liquid paraffin • Ultimate limit: scattering caused by thermally-induced Shape fluctuations Lai et al. PRA 1990 • We can move to the visible to increase Q
WGMs in the visible region (660 nm) As expected, the WGMs in the visible range exhibit narrower resonances WGM ring Q-factor 107 HWHM 10 -20 MHz
Detection of metallic NPs: preliminary results Au nanop 10 nm dia = t/γ = n. L/(c(αL+A)) M. R. Foreman, S. Avino, R. Zullo, H. -P. Loock, F. Vollmer, and G. Gagliardi, Eur. Phys. J. , in press
Conclusions Summary • • • Liquid droplet as micro-resonator Near-IR laser locked to a WGM with PDH CRDS with locked laser Oil absorption measurements NPs detection in the visible by WGM broadening Outlook • Use other liquids for droplets, different wavelengths • Improve method of analyte delivery • Many other experiments possible…
Acknowledgments INO «Optical Sensors» group Saverio Avino (research fellow) Pietro Malara (research fellow) Antonio Giorgini (post-doc) Rosa Zullo (post-doc) Maria. Luisa Capezzuto (grad student) Anika Krause (grad student) Long-term Scientific collaborations: Short-term visitors: H. P. Loock, Queen’s University, Kingston (CAN) Matthias Fabian (Limerick University, IR) Helen Waechter (Queen’s University, CAN) Financial support - University and Research Ministry PON progr - CNR (RSTL project) - CNR Short-term mobility program Dept Chemistry, Queen’s Univ, Kingston (CAN) Max Planck, Erlangen (D) Koch University, Istanbul (TK) Lawrence Berkeley National Laboratory, Molecular Foundry (USA) Central Glass and Ceramic Research Inst. -CISR, Calcutta (IN)
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